Curable silicone rubber compositions which cure to form a silicone rubber elastomer are well known and widely used as a gasket material for electric and electronic parts, a potting material, a coating material, a shaping material such as a molding material, a wire-covering material and automobile parts, on account of good properties of the compositions such as weathering resistance, heat resistance, and electrical insulation property. Curable silicone rubber compositions have siloxane bonds which are inherent in a silicone. A siloxane bond is ionic and, therefore, the curable silicone rubber compositions cannot exert the excellent properties of silicones such as chemical resistance, for instance, acid and alkali resistance, water-proof property and permeability for gas in extremely severe environments such as a hot and humid environment.
As a countermeasure for the afore-mentioned problem, a polymer is known where a part of its siloxane bonds is a silethylene bond (Patent Literature 1) or a silphenylene bond (Patent Literature 2). However, the synthesis of these polymers and, accordingly, their mass production, are difficult and, therefore the polymers are expensive. Therefore, the polymers have been commercialized only in special applications or in special fields.
As a precursor for silicon carbide ceramics, a polydiarylsilmethylenes having silmethylene bonds are known from Patent Literatures 3 to 5. These polymers are crystalline, thermoplastic silicon polymers having a high melting point and are good in heat resistance, insulation property, electrical properties, chemical resistance and water-proof property. However, the polymer is poor in processability by molding and, therefore, is not practically used. Various efforts have been made to improve the processability by molding. For instance, Patent Literature 6 describes a mixture of a polydiarylsilmethylene with a silicone polymer. Patent Literatures 7 and 8 describe mixtures of a polydiarylsilmethylene with a polyalkylsilmethylene. Patent Literature 9 describes a method for preparing a film of a polydiarylsilmethylene on a substrate where a film of disilacyclobutane is formed on a substrate and is ring-opening polymerized using fine particles of metal. However, the thermoplastic diarylsilmethylene polymer is highly crystalline and, therefore, is difficult to be synthesized. Accordingly, the diarylsilmethylene polymer is expensive and bad in processability. Therefore, research has been made to adopt the diarylsilmethylene polymer as a precursor for silicon carbide ceramics. However, a thermosetting composition is not known which takes advantages of the properties as a polymer.